1. Field of the Invention
The present invention relates to printing apparatus and methods, more particularly to improved apparatus for printing single- or multiple-color copies using digital spark-discharge recording technology.
2. Description of the Related Art
Traditional techniques of introducing a printed image onto a recording material include letterpress printing, gravure printing and offset lithography. All of these printing methods require a plate, usually loaded onto a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. Gravure printing plates, in contrast, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the plate by a doctor blade or similar device prior to contact between the plate and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording material; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other copying medium. In typical rotary press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
The plates for an offset press are usually produced photographically. In a typical negative-working subtractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive oxide surface coated with a photopolymer. Upon exposure to light or other radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), and these non-cured areas become oleophobic and/or hydrophilic.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required, each such plate usually being made photographically as just described. In addition to preparing the appropriate plates for the different colors, the operator must mount the plates properly on the plate cylinders of the press, and coordinate the positions of the cylinders so that the color components printed by the different cylinders will be in register on the printed copies. Each set of cylinders associated with a particular color on a press is usually referred to as a printing station.
In most conventional presses, the printing stations are arranged in a straight or "in-line" configuration. Each such station typically includes an impression cylinder, a blanket cylinder, a plate cylinder and the necessary ink (and, in wet systems, water) assemblies. The recording material is transferred among the print stations sequentially and in register, each station applying a different ink color to the material to produce a composite multi-color image. Another configuration, described in U.S. Pat. No. 4,936,211 (co-owned with the present application and hereby incorporated by reference), relies on a central impression cylinder that carries a sheet of recording material past each print station, eliminating the need for mechanical transfer of the medium to each print station.
With either type of press, the recording medium can be supplied to the print stations in the form of cut sheets or a continuous "web" of material The number of print stations on a press depends on the type of document to be printed. For mass copying of text or simple monochrome lineart, a single print station may suffice. To achieve full tonal rendition of more complex monochrome images, it is customary to employ a "duotone" approach, in which two stations apply different densities of the same color or shade. Full-color presses apply ink according to a selected color model, the most common being based on cyan, magenta, yellow and black (the "CMYK" model). Accordingly, the CMYK model requires a minimum of four print stations; more may be required if a particular color is to be emphasized. The press may contain another station to apply spot lacquer to various portions of the printed document, and may also feature one or more "perfection" assemblies that invert the recording medium to obtain two-sided printing.
A number of difficulties attend both the platemaking and ink-transfer stages of printing. The photographic process used to produce conventional plates is time-consuming and requires a facility and equipment adequate to support the necessary chemistry. To circumvent this process, practitioners have developed a number of electronic alternatives to plate imaging, some of which can be utilized on-press. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic-radiation pulses, produced by one or more laser or non-laser sources, that create chemical changes on plate blanks (thereby eliminating the need for a photographic negative); ink-jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing "dots" which collectively form a desired image.
While these digital platemaking technologies have alleviated many of the disadvantages associated with more traditional approaches, they are not free from drawbacks of their own. Such drawbacks are described in U.S. Pat. No. 4,911,075 (co-owned with the present application and hereby incorporated by reference).
Presses must also be provided with mechanical assemblies for maintaining and correcting registration among the images applied by the various print stations In the case of an in-line press, it is necessary to employ very accurate paper-feeding and paper-transfer mechanisms, as well as precision gearing, to assure consistent positioning among print stations. The press should also allow for correction of misregistrations by adjustment of the relative positions of the plate cylinders to maintain proper rotational, axial and skew-orientation phase; so long as the paper is fed and transferred accurately among print stations, such positioning corrections will correct misregistrations on a consistent basis.
The mechanical difficulties of maintaining registration are ameliorated, but not eliminated, if the plate is to be imaged on-press. In this case, mispositioning due to improper mounting of the finished plate onto the plate cylinder is effectively overcome. However, in a multi-station press, it becomes necessary to maintain registration among plate cylinders during both the plate-imaging and printing stages. Specifically, not only must the print stations apply ink in register with one another, but each individual plate-imaging system must be coordinated both with its own plate cylinder (which holds the plate to be imaged) and with one another so as to maintain, consistent plate orientations.
The ink flow at each print station must also be accurately regulated, as well as remain adjustable to accommodate different ink densities or produce a desired color correction on the final printed copy. As discussed in U.S. Pat. No. 4,058,058, a press may be equipped with a number of electrically controlled ink-regulating screws or keys distributed across the press to regulate the amount of ink that the ink fountain at each print station applies to the plate cylinder at that station. These regulators may be controlled manually or, to some extent, with the assistance of computer equipment. In some publishing systems, for example, the color separations prepared from each page mock-up are scanned and stored digitally as proofs; hard copy produced by the press is similarly scanned, and digitally compared with the mock-up proofs to determine the necessary ink-regulation adjustments. Thus, at present, an operator must devote time and/or skilled judgment to determine the settings of ink regulators.